Early in normal human pregnancy, maternal plasma osmolality decreases and plasma volume increases. A failure to achieve adequate plasma volume expansion is associated with poor pregnancy outcomes, including reduced amniotic fluid (AF) volume (oligohydramnios). Recent experiments have established that administration of a synthetic hormone (dDAVP) which concentrates maternal urine results in expansion of maternal plasma volume, development of maternal and fetal plasma hypoosmolality fetal urine production and increased AF volume. Through this four part project we hope to: develop a simplified index of maternal plasma osmolality using intravenous saline infusions and measurements of maternal hormones; correlate the association of maternal plasma osmolality with maternal plasma volume in a series of normal pregnant patients throughout gestation; examine the degree of dDAVP placental transfer and examine the effects of acute dDAVP induced plasma hypo-osmolality on fetal urine flow and AF volume in patients with reduced AF volume. These studies will aid in the development of markers of prediction and therapeutic approaches to the prevention and treatment of human oligohydramnios.